Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
Solar cells are one example of a device that uses silicon workpieces. Any reduction in the cost to manufacture solar cells or any efficiency improvement to solar cells would have a positive impact on the implementation of solar cells worldwide. This will accelerate the adoption of solar cells as a clean energy technology.
There are several different solar cell architectures. One common design is the selective emitter (SE) solar cell. A SE solar cell has lightly-doped regions to enable lower current recombination as well as heavily-doped regions that enable current collection through low resistance contacts and minimize resistive losses. There are several techniques for the fabrication of such SE solar cells using doped inks, selective diffusions, laser doping, or patterned ion implantation. However, all of these techniques rely on some method of masking to enable selective doping on the SE solar cell surface.
The front surface of solar cells is typically textured to minimize the reflective losses from the surface. A pyramidal texture on the surface may result from an anisotropic etch of a monocrystalline silicon substrate. This etch may use a mixture of KOH and isopropyl alcohol in one instance. FIG. 1 is a perspective view of implanting a textured workpiece in a first embodiment. An ion beam 100 is directed at the textured surface 101 at a normal angle of incidence (i.e., perpendicular to the overall plane formed by the textured surface 101 or plane of the solar cell or other workpiece). Each of the four facets of the textured surface 101 illustrated in FIG. 1 is doped approximately equally. Thus, each facet receives approximately 57.7% of the nominal dose. Multiple implant steps may be required to dope a solar cell with both high and low dopant concentration regions. Additional steps, such as laser ablation or resist patterning and stripping, also may be needed. Such steps may decrease throughput or increase manufacturing costs. Therefore, there is a need in the art for an improved method of solar cell manufacturing and, more particularly, an improved method for ion implantation of solar cells.